/Engine/Plugins/Experimental/MeshModelingToolsetExp/Source/MeshModelingToolsExp/Public/MeshSelectionTool.h
Grow the active selection to include any triangle connected via shared edges (ie flood-fill)
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(CallInEditor, Category=SelectionEdits, Meta=(DisplayPriority=5)) void FloodFill()